urlapi: support UNC paths in file: URLs on Windows

- file://host.name/path/file.txt is a valid UNC path
  \\host.name\path\files.txt to a non-local file transformed into URI
  (RFC 8089 Appendix E.3)

- UNC paths on other OSs must be smb: URLs

Closes #7366

tests/libtest/lib1560.c

@@ -187,6 +187,10 @@ static const struct testcase get_parts_list[] ={
   {"file:///C:\\programs\\foo",
    "file | [11] | [12] | [13] | [14] | [15] | C:\\programs\\foo | [16] | [17]",
    CURLU_DEFAULT_SCHEME, 0, CURLUE_OK},
+  {"file://host.example.com/Share/path/to/file.txt",
+   "file | [11] | [12] | [13] | host.example.com | [15] | "
+   "//host.example.com/Share/path/to/file.txt | [16] | [17]",
+   CURLU_DEFAULT_SCHEME, 0, CURLUE_OK},
 #endif
   {"https://example.com/color/#green?no-red",
    "https | [11] | [12] | [13] | example.com | [15] | /color/ | [16] | "